I would love a video tutorial of how to make frosting roses directly on the cake :))))
Go to youtube and type in buttercream flowers. You will see videos after videos showing how to make flowers and also showing the tubes used. Also you can type in buttercream roses. Many make them on flower nails then put them in freezer for about 5-10 minutes so you can pick flower up and place on top of cake so just search the videos and you will find some who make them directly on top of the cake. Have fun because you will enjoy watching the videos.
@kathytroeger why do you want to pipe them directly on the cake? I think it's actually faster and easier to pipe them onto a flower nail or stick and then transfer them to the cake. If you pipe them directly onto the cake you'll likely smash your roses as you continue to pipe new ones, or you won't be able to pipe them very close together.
Here are 2 tutorials for piping, with a nail and a stick: